The immune system consists of a variety of white blood cells and proteins that attack foreign invaders such as bacteria and viruses. The human immune system must distinguish harmless from harmful organisms.

The first line of defence is the skin. The outer layer of the skin is dry and not a hospitable environment for bacteria, fungi, and other pathogens.

There are two main types of defences for survival in the human body:

Specific Immunity

·The ability to resist a disease after being exposed to it in the past

·Includes a wide variety of cells that recognize foreign substances and act to neutralize or destroy them

·Each individual develops a unique immune system that is able to deal with a variety of possible infections

·Also includes natural killer cellsother white blood cells that carry out phagocytosis targets are body cells that have become cancerous or infected by viruses

B-cells and T-cells work together to attack invaders. Cellular immunity is performed by T-cells and antibody immunity is performed by B-cells. Both systems are controlled by T-cells and are initiated by the action of macrophages.

Macrophages engulf antigens, and the foreign cell is moved to the surface of the macrophage. T-cells and B-cells have specialized receptors on their surface that match specific antigens.

Cellular Immunity

In cellular immunity, T-cells that have attached to a macrophage with a particular antigen goes through a process of rapid cell division. The antigen can be either a invading bacteria, human cells that have been infected by a virus, or cancer cells. This process produces a number of different types of T-cells:

Cell Type:

Function:

Helper T-cells

·Acts as a guard to identify foreign invading substance

·Stimulates other macrophages, B-cells, and T-cells

Suppresser T-cells

·Slows down or stops the process of cellular immunity

Memory T-cells

·Remains in the bloodstream to promote a faster response if the same foreign antigen reappears

Killer T-cells

·Punctures cell membranes of infected cells

Antibody Immunity

Antibody immunity begins when a macrophage ingests a foreign cell and moves the antigen to its cell membrane - the same way cellular immunity begins. However, in antibody immunity, T-cells trigger specialized B-cells to undergo rapid cell division. The dividing B-cells form plasma cells and memory B-cells. The plasma cells produce large quantities of antibody proteins that are matched to the foreign antigen. Antibodies attack and mark antigens for macrophage destruction. Memory B-cells stay in the blood to speed up the response if the same antigen reappears, much like the memory T-cells.

Allergies

Allergies occur when your immune system mistakes harmless cells, such as pollen or mould, for harmful invaders. There are two types of allergic reactions: immediate and delayed. An immediate reaction occurs within seconds of exposure to the allergen and usually disappears within 30 minutes. These reactions release histamines, which are chemicals that increase the permeability of blood vessels, making the area red and swollen. A delayed reaction is slower and lasts for a longer period of time. These reactions are set off by T-cells that have been sensitized by previous contact with the allergen.